Generally, a clutch disc arrangement of the type having a first and a second clutch hub portions, a suitable clearance is only allowed between both clutch hub portions without provision of any separate member to be interposed therebetween. While the arrangement has been used for extended periods of time, this incurs wearing and rusting of the sliding portions between both clutch hub portions, and possibly gives rise to desired hysteresis in the "angle of torsion-torque" curve, these being counted as drawbacks. When the contact areas of the sliding portions are reduced to decrease the magnitude of the hysteresis, they are apt to wear away due to high contact pressures, which is also counted as a drawback.
This type of clutch disc may generally be of the following structure, i.e., designed to make rotating engagement between a hub flange of a clutch hub and a disc plate through damping means and friction means (elements). The hub flange comprises an inner flange portion (first hub portion) and an outer flange portion (second hub portion) disposed to be rotatable by a given angle relative to the inner flange and with an angular clearance in the rotating direction. The inner flange portion is engaged with the outer flange portion through damping means (damping springs) in damping relation for relative rotation. A first friction member is disposed operatively between the inner and outer flange portions for acting on both portions, and the outer flange portion is engaged with the disc plate through damping means and friction means having varied characteristics in such a manner that any rotational shocks are damped or absorbed. At least one damping means is arranged in one of windows and/or notches (hereinafter referred to as "window/notches" formed in any one of the outer flange portion and the disc plate, and intended to be engageable within the other corresponding one of circumferentially larger window/notches provided in the other of the outer flange portion and the disc plate.
This type of clutch is called the two-divided hub type wherein, within the limits of an angular clearance allowed between the inner and outer flange portions, torque is transmitted from the inner flange to the outer flange through damping means (springs) disposed between the inner and outer flange portions. However, once the engagement of both flange portions is achieved, a plurality of springs disposed therearound work additionally in a successive manner, so that the torque is transmitted to the portion of the clutch disc having friction linings in a damping manner. In this type of clutch, it constitutes a first-range changeover point of hysteresis width when the engagement of the inner and outer flange portions is achieved. In the prior art, second, third and further-range changeover of hysteresis width is effected by means of a control plate, which works (i.e., cause changeover of hysteresis width) in operative correspondence with the commencement of action of damping means (first, second, third . . . damping springs) that are disposed in the mutually associated window/notches provided in the outer flange portion and the disc plate to take part in damping transmission of torque for the second, third and further-stage (torque stage).
On the other hand, it is desired to vary the magnitude of hysteresis width in the torque stages corresponding to low, moderate and high speed ranges, if required, with a view to reducing or eleminating various sorts of noises that the driving system generates. These noises practically include so-called transmission rattle noise generated when the tooth surfaces of two gears meshing with each other based on back lash collide with each other due to variation of the engine revolutional speed at the neutral state of transmission, so-called differential tapping noise, viz., back lash noise generated from differential gears during running and starting, and so-called booming or humming noise generated during constant or accelerated running (considered to be generated from the overall driving system), etc.
In the prior art, however, since the width of hysteresis at the respective stages is determined in operative association with the operation of the respective springs operable at the respective stages set up according to the requirements for the torque transmission curve, it has been difficult to determine the width of hysteresis independently of such determination, i.e., from a different point-of-view. In particular, the width of hysteresis at the second and third ranges should be determined independently of the changeover points of torque stages on the torque transmission curve so as to reduce or eliminate various sorts of noises.